1. Field of the Invention
The present invention relates to an electrochemical sensing cell, and particularly to an improved housing for such a cell.
2. Description of the Prior Art
Concern for the quality of the air we breathe has led to mandatory requirements for monitoring of air contaminants. Federal and state environmental protection agencies have imposed such monitoring requirements both to ensure compliance with statutes establishing maximum pollutant levels, and to provide a data base for evaluation of the contamination problems associated with certain industries, power generation plants, motor vehicle exhausts and other pollutant sources. To accomplish such monitoring, the need exists for simple, inexpensive, accurate and trouble-free monitors, and it is a principle object of the present invention to provide an improved electrochemical sensing cell useful in such an instrument.
An electrochemical sensing cell is a device which generates an electrical current only in the presence of the pollutant being measured. The magnitude of this current is proportional to the pollutant concentration, which may be indicated by a meter connected to the output of an amplifier which amplifies the current from the sensing cell.
An electrochemical sensing cell incorporates two electrodes, one called a sensing electrode and the other called a counterelectrode, immersed in an electrolyte. When the pollutant gas contacts the sensing electrode, reactions occur which cause a current to flow in a circuit comprising a counterelectrode, the electrolyte, the sensing electrode and an external lead connecting the sensing electrode back to the counterelectrode. The magnitude of this current is proportional to the pollutant concentration. By the appropriate selection of counterelectrode and electrolyte materials, in conjunction with external biasing, the sensing cell may be made selective to a particular gas species. For example, the sensing cells disclosed in the Chand/Shaw U.S. Pat. Nos. 3,622,487 and 3,622,488 are intended selectively to detect nitrogen oxide and sulfur dioxide respectively.
The basic electrochemistry of such sensing cells is well known. Depending on the species to be detected, either oxidation or reduction occurs at the sensing electrode, and the complementary reaction occurs at the counterelectrode. For example, to detect hydrogen sulfide (H.sub.2 S), oxidation occurs at the sensing electrode, which preferably comprises a noble metal such as gold or platinum. Electrochemical reduction occurs at the counterelectrode, which may comprise lead in an electrolyte of sulfuric acid. Preferably, the counterelectrode is non-polarizable, so that it does not change its potential when current is passed through it. This permits the counterelectrode also to function as a reference electrode. That is, the reduction potential associated with the reduction reaction at the counterelectrode is a fixed potential against which the oxidation potential at the sensing electrode may be referenced. Since these potentials are known per se, an appropriate bias voltage may be selected to ensure that only H.sub.2 S gas is oxidized at the sensing electrode. The oxidation/reduction potentials are set forth in standard chemical texts such as that by Wendell Latimer, entitled OXIDATION-REDUCTION POTENTIALS.
There are many practical problems associated with packaging electrochemical sensing cells. Certain of these concern the sensing electrode. Cell operation requires that the sensing electrode be in contact with the electrolyte so that the requisite oxidation or reduction can occur at the sensing electrode with appropriate current flow through the electrolyte. At the same time, the sensing electrode must be exposed to the gas being analyzed. That is, molecules of the gas species being detected must be able to reach the sensing electrode where they are oxidized or reduced. These two requirements of (a) contact with the electrolyte and (b) exposure to the gas being sensed place conflicting demands on the sensing electrode. If the area of exposure to the gas is large, the opportunity exists for excessive evaporation of the electrolyte. Furthermore, leakage of the electrolyte through the sensing electrode also may be a problem. One approach of the prior art, utilized in the above mentioned Chand/Shaw patents and in the U.S. Pat. Nos. 3,429,796 to Lauer and 3,755,125 to Shaw, involves the use of a thin membrane covering the sensing electrode. The membrane provides a liquid tight seal that prevents leakage and reduces evaporation of the electrolyte. The membrane material is porous to the gas being sensed, which passes through the membrane to the sensing electrode.
Another requirement of the sensing electrode is that it have a large effective surface area. In the above cited Chand/Shaw and Lauer patents, this was accomplished by using a micromesh screen of gold or platinum as the sensing electrode. An alternative approach utilizes fine particles of the noble metal bound in a polymeric dispersion. This approach is shown e.g., in the German Auslegeschrift No. 1,233,173 to Guthke and Habermann. There, finely powered metals are bound in a porous structure of permutite and a plastic such as polystyrol. The electrolyte diffuses through this porous structure so as to immerse completely the sensing electrode metal particles. The gas being monitored dissolves through the diffused electrolyte to contact the sensing electrode metal. Very good sensitivity is achieved because of the large effective surface area of the powdered metal. Indeed, in some applications it may be desirable to decrease the sensitivity, and this can be accomplished by controlling the amount and density of the permutite in the dispersion.
Electrolyte evaporation also may be a problem in sensing cells of the type which employ separate counter- and reference electrodes. For example, in one known type of CO monitor, the sensing electrode is located at one end of a cylinder which houses the electrolyte. The reference electrode and counterelectrode are situated at the other end. The counterelectrode requires a supply of oxygen, and this is obtained by exposing the counterelectrode to air. As a result, the evaporation problem described above occurs both at the sensing electrode at one end of the cell and at the counterelectrode at the other end. A much higher evaporation rate results, with concomitantly short sensor lifetime. That is, the electrolyte must be replaced in the cell at relatively short intervals.
Thus, another object of the present invention is to provide an electrochemical sensing cell in which electrolyte evaporation is minimized, so that the cell can be used for long periods of time before the electrolyte must be replenished. Another object of the present invention is to provide such a sensing cell employing a dispersion-type sensing electrode in a housing configuration that minimizes both leakage and evaporation of the electrolyte. A further object is to provide such a cell in which replenishment of the electrolyte, when required, can be accomplished very simply, without major disassembly of the cell.
Another problem associated with electrochemical sensing cells concerns sloshing of the electrolyte. This is undesirable since it may result in intermittent contact between the electrolyte and the sensing electrode. This of course would result in intermittent operation or erroneous measurements. Yet for portability, it is necessary that the cell remain operative despite movement which may occur if the instrument is used in an aircraft, automobile or boat. Another object of the present invention is to provide an electrochemical sensing cell in which the electrolyte is immobilized so as to eliminate the sloshing problem.
Another problem resulting from the structural configuration of the sensing cell concerns the pressurization requirements for the gas being analyzed. Certain cell configurations are such that the gas cannot be pressurized above or below the environmental ambient pressure. Such excess pressure or suction could distort or damage the sensing electrode. With such cells, a sample bag must be employed to collect some of the gas being analyzed and to provide this to the cell at the environmental ambient pressure. A further object of the present invention is to provide an electrochemical sensing cell which can operate with a gas source that is pressurized either above or below the environmental ambient level.